As with all automotive parts (be it oem or aftermarket), there is always a chance of a part failing, especially if it is an internal performance part (not a bolt-on) that is subject to the kind of stresses that a transmission needs to go through. It doesn't matter who the manufacturer of the part is (every manufacturer will have people running the parts no problem, whilst others have the parts failing on them); what matters is if the manufacturer will still be there to help you "after" they've taken your money.
I believe we are the only manufacturer to offer a full 1 year warranty on gears. Transmission internals are so easy to break, and can be due to so many different variables (one of which is the standard "the customer is never at fault, so blame it on the manufacturer") that most will not offer any warranty whatsoever once the gears are installed and run in your transmission, regardless of who is at fault. You could spend £2500 with Hewland for a custom final drive, and if it breaks 10 miles down the road, you are sh*t out of luck. No one would ever say Hewland products are not good though (they do supply F1 transmissions after-all). We offer it because we stand behind our product 101% (and also because we manufacturer these in the hundreds, so will always have spares on the shelf, unlike our competitors who would have to split their "kits", leaving them with spare parts they can't sell)
We are definitely the only manufacturer though to offer a 30-day money back guarantee on our final drives and LSD's
If you still have doubts, please feel free to PM me and I can discuss further about our manufacturing processes to assure you of our quality and customer service.